A head cover of an internal combustion engine is formed with an oil inlet through which lubricating oil is supplied to a valve mechanism, etc., and a cylindrical cap attachment boss to which an oil filler cap for closing the oil inlet can be detachably attached is formed around the oil inlet. Thus, the cylindrical cap attachment boss is formed on the head cover as a protrusion, and the oil inlet is formed inside of the attachment boss as an opening.
Some of such head covers are formed with an oil outflow prevention wall (barrier) that surrounds the cap attachment boss to define an oil trap portion around the cap attachment boss in order to prevent the oil that has been accidentally poured or spilt outside of the cap attachment boss when oil is supplied through the oil inlet from spreading over a wide area to contaminate a considerable portion of the head cover (see Japanese Utility Model Application Laid-Open Publication No. 61-175514, for example).
In some of the vehicles such as automobiles, an internal combustion engine (cylinder block) may be mounted transversely in such a manner that a cylinder axis is inclined with respect to a vertical line. Such an engine may be called an inclined engine. In the inclined engine, an upper surface of a cylinder head to which the head cover is fastened is inclined with respect to a horizontal plane, and hence the head cover is also inclined with respect to the horizontal plane in the installed state.
In this regard, in the conventional head cover, the upper end of the oil outflow prevention wall extends horizontally when the cylinder head is positioned horizontally. This can cause a problem in the inclined engine that in a lower side of the inclination, the oil spilt from the cap attachment boss can easily get over the oil outflow prevention wall to outside.
The higher the oil outflow prevention wall is, the harder the oil flows over the oil outflow prevention wall to outside. However, the higher the oil outflow prevention wall is, the more difficult it becomes to access the oil inlet to supply the oil and to attach or detach the oil filler cap to and from the attachment boss. Further, a higher oil outflow prevention wall can lead to a greater overall height of the engine and thus may compromise an advantage of the inclined engine that the inclined engine can assume a smaller height when installed in the vehicle.